1. Field of the Invention
The present invention relates generally to superconductor materials formed from powdered metal oxides, and particularly relates to providing improvements on long length high temperature superconductor wires and tapes such as Bismuth-2223, Bismuth-2212 and Thallium-1234, such improvements including reduced microcracks, improved critical current densities, and higher flux pinning.
2. Description of Prior Art
Superconductor wires and tapes have, for many years, been fabricated according to a conventional technique known as the "powder-in-tube" method. One example of such method is described in U.S. Pat. No. 4,906,609, although more recently, the use of silver tubes has been preferred for forming such wires and tapes. Typically, the method involves utilizing initial starting materials and providing for a series of grinding and heat treatment operations to produce a powdered mixture which is then packed into the silver tube. Thereafter, a series of cold working operations including drawing and rolling are performed to thereby provide a desired thickness of the wire or tape. An intermediate heat treatment is introduced followed by rolling to final thickness and thereafter, annealing. The wire or tape is then formed into a magnet coil and the final heat treatment operation is performed on the magnet coil.
A number of problems have been found with the final magnet coil thus produced. One problem relates to the introduction of microcracks. An illustration of a typical microcrack can be seen in FIG. 2. Generally, such cracks are introduced during the rolling operations while producing the final thickness. For example, we have found that the critical current density (Jc) when pressed to final thickness rather than rolled and heat treated repeatedly will show improvement as a function of time. However, under similar conditions of temperature and time the Jc for repeated rolling/heat treatments to produce the final thickness does not show improvements. In contrast, the Jc decreases as the rolling steps increase. The graph of FIG. 1 illustrates the differences achieved when pressing as opposed to rolling to achieve the final thickness. Plots 1 and 2 illustrates the Jc's obtained versus total heat treatment time for wires pressed at varied pressures and at 18 kbar pressure, respectively. FIG. 1 illustrate the Jc's obtained versus total heat treatment time for wires rolled twice and three times, respectively. During the rolling operations we find increased development of microcracks, which is a consequence of the non-uniform application of stresses from the rolls. The enhancement of the critical current density by using uniaxial pressing instead of cold rolling, to produce the final thickness of the wire or tape, prior to annealing is disclosed in Applied Physics Letters, Volume 60, Number 4, Jan. 27, 1992 pages 495-497.
The method of starting with a composition of material having an excess of bismuth and calcium and utilizing the "powder-in-tube" method of placing ground powder into silver tubes, drawing the powder into round wires and then making it into tapes by pressing or rolling is disclosed in The Japanese Journal of Applied Physics, Volume 30,Number 12B, December 1991, pages L2083-L2084. Although this reference states that Jc's of up to 5.4.times.10.sup.4 A/cm.sup.2 at 77.3K in a zero magnetic field and 8.9.times.10.sup.4 A/cm.sup.2 at 4.2K in a magnetic field of 23T were obtained, these Jc's can only be obtained on very small lengths of wire. This method is, furthermore, not commercially practiced. Performing such a method on wires having a length which is not very small causes severe discontinuities. Severe discontinuities refers to variations in thickness induced by the type of pressing disclosed along with the possible introduction of microcracks. The pressing disclosed is also not an in-line pressing to improve flux pinning sites, this pressing is to obtain the final thickness and shape of the wire. This method, therefore, would create severe discontinuities and is, therefore, not applicable for producing a wire having a length which is more than negligible.
It has also been suggested to deform Bi-2223 tapes by hot rolling. Such was introduced by R. Flukiger, A. Perin and E. Walker from the Universite de Geneve in Switzerland at the Materials issues in High Temperature Superconductivity Spring Symposium T on Apr. 12-16, 1993. A deformation technique was developed to enhance the degree of texturing as well as the density by hot rolling. A prototype rolling machine was constructed with rolls of 80 mm diameter that can be heated up to 800.degree. C. Various problems were encountered, each requiring separate solutions. The most important are: a) the flow of the Ag sheath at elevated temperatures which leads to enhanced sausaging, b) sticking of the Ag sheath to the heated rolls, and c) the precise determination of the tape temperature between the two rolls.
Additionally, problems with the final coil can result from the appropriate starting materials. One prior art method is to initially utilize ground powders of nonsuperconducting phase as Bismuth-2212 as part of the initial matrix powder inserted into the silver tube. Such has been described by way of example in U.S. Pat. No. 5,057,486.
While there is continued progress at providing increased critical current densities for such high Tc superconductor materials, further improvements are still needed. Furthermore, when providing long length coils of such wire and tape, the microcracks introduced into the high Tc materials continue to detract from the improved current densities needed in such materials.
In addition to the Bismuth based high Tc materials, similar problems have been found in the Thallium based high Tc materials, such as Thallium-1234.
Accordingly, a method of healing such microcracks is needed, and at the same time of producing improved methods for providing high Tc Bismuth based and Thallium based superconductors having improved critical current densities as compared to present known superconductors.